def prost(x):
    k = 0
    for d in range(1, x+1):
        if x%d==0:
            k+=1
    if k==2:
            return True
    else:
            return False
maxx = 0
num=int(input())
if num>= 1000 and num <= 9999:
    n1 = str(num)
    for cifra in range(len(n1)):
        if prost(int(n1[cifra])):
            maxx = max(maxx,int(n1[cifra]))
    for chislo in range(len(n1)-1):
        if prost(int(n1[chislo]+n1[chislo+1])) == True:
            maxx = max(maxx,int(n1[chislo]+n1[chislo+1]))
    for chislo in range(len(n1)-2):
        if prost(int(n1[chislo]+n1[chislo+1]+n1[chislo+2])) == True:
            maxx = max(maxx,int(n1[chislo]+n1[chislo+1]+n1[chislo+2]))
    for chislo in range(len(n1)-3):
        if prost(int(n1[chislo]+n1[chislo+1]+n1[chislo+2]+n1[chislo+3])) == True:
            maxx = max(maxx,int(n1[chislo]+n1[chislo+1]+n1[chislo+2]+n1[chislo+3]))
    
    print(maxx)
else:
    print(0)
            
        
